This isn't just some random targetWhy expend so much effort? Sure, your original prey decided to hole up in someone's house. Why not move on to the next house? Or the next guy wandering the streets? Unless, of course, this isn't just a random bystander. Two likely scenarios:
America is a one-party stateFrom going on their website  one can see there is no reference to any political party other than the one promoted all over the pages: the New Founders of America, NFA. There is, though, reference to unity, and strength through Purging. The corruption, stagnation, and decay of multi-partisan democratic government is passive-aggressively denounced, and there are hints of a mass political reorganization under the NFA, which they refer to as the "Great Integration". This means America as we know it is dead, though they still use the familiar symbols of American democracy, such as the same federal government, the state governments, the Statue of Liberty, the Constitution, etc. America has traded freedom for safety, forgetting Benjamin Franklin's words, and making a mockery of the original America. The Party symbol is revered as almost a new national symbol: the blue baptisia flower.
The guy who wanted to kill his boss ended up getting killed by said boss.
The leaders of this society have a deal with the elder gods, sort of like The Cabin in the Woods, except on a more massive scale.In return for a massive human sacrifice on Purge night they get prosperity for the rest of the year.
One of the members of the New Founding Fathers is Princess MorebucksC'mon! She started The Purge in Townsville before!
The Purge is a pseudo-sequel to The Running Man
There are a whole lot of 5-and-under children with birthdays in December.
The first Purge or two weren't murder sprees.For every American who wanted to get their kill on, there must have been 5-10 who heard "nothing is forbidden, everything is permitted" and thought "gee, I've always wanted to smoke some dope/hire two hookers/drive 90 mph on the highway" or other similar crimes. That first night, plenty of people were out doing a wide cross-section of petty crimes and misdemeanors. But the next year, when the bona-fide whackos are sure that the Purge isn't just some trick to get people to incriminate themselves, the blood starts to really flow, with the petty offenders providing a target-rich enviroment for your spree killers and oppourtunity rapists. The 364 days after that are when people really start preparing for a lockdown, leading to the situation as of the film's Purge.